Chemistry 4.1 What are chemical reactions? — Reactants, Products and Chemical Change

Section 1 of 10What a chemical reaction is

• Chemical reaction = a change that makes new substances, in which atoms are rearranged and join together differently.
• Not easily reversed
  • at the end it is difficult to get back to the starting substances.
• Energy
  • all chemical reactions transfer energy to or from the surroundings.
• Everywhere
  • chemical reactions happen in places such as engines, kitchens and inside your body.

Section 2 of 10Reactants and products

• Reactants = the starting substances in a chemical reaction.
• Products = the substances that are made.
• Word equation
  • an arrow means "react to make", with reactants on the left and products on the right.
  • Hydrogen + oxygen → water.
• Different properties
  • the products have different properties from the reactants because their atoms are rearranged into new substances.

Section 3 of 10Signs of a chemical reaction

• Flames or sparks.
• A smell, sweet or foul.
• Substances getting hotter or colder.
• A loud bang or gentle fizzing.
• Colour change, gas, new solid (precipitate) or energy change.
• Physical changes
  • dissolving and changing state are not chemical reactions.

Section 4 of 10Burning magnesium

• Reaction
  • a piece of shiny magnesium held in a flame burns brightly, joins with oxygen from the air and leaves white ash.
• Reactants and product
  • the reactants are magnesium and oxygen, and the one product is magnesium oxide (the white ash).
• Different properties
  • the reactants and the product look different.

Section 5 of 10Magnesium and acid

• Observation
  • magnesium added to hydrochloric acid bubbles.
• Product
  • the bubbles are hydrogen gas, one of the products.
• Displacement
  • magnesium reacts with iron oxide to make magnesium oxide and iron.

Section 6 of 10Working safely

• Flammable
  • magnesium burns quickly and has the hazard symbol "flammable".
• Hazard symbol = a symbol that shows how a substance can cause harm, and hazard symbols are the same everywhere.
• Reducing risks
  • keep the roll of magnesium away from flames.
  • Hold the magnesium with tongs because it burns with a hot flame.
  • The bright white flame can damage eyesight, so do not look at it directly.

Section 7 of 10Types of reaction

• Combustion
  • a fuel burns in oxygen, giving out energy, for example methane + oxygen → carbon dioxide + water.
• Thermal decomposition
  • one substance breaks down on heating, for example calcium carbonate → calcium oxide + carbon dioxide.
• Oxidation
  • a substance gains oxygen, for example magnesium + oxygen → magnesium oxide.
• Displacement
  • a more reactive metal pushes out a less reactive one from a compound.
• Neutralisation
  • an acid and alkali react to form a salt and water.

Section 8 of 10Energy changes

• Exothermic = a reaction that gives out energy to the surroundings and the temperature rises, such as burning, respiration and neutralisation.
• Endothermic = a reaction that takes in energy and the temperature falls, such as thermal decomposition and photosynthesis.
• Uses
  • hand warmers use exothermic reactions and instant cold packs use endothermic ones.

Section 9 of 10Chemical or physical change

• Chemical change
  • new substances form and it is usually hard to reverse, for example burning paper.
• Physical change
  • no new substances form and it can be reversed easily, such as melting, boiling or dissolving.

Section 10 of 10Speed of reactions

• Rusting is slow, burning is fast and an explosion is very fast.
• Everyday reactions
  • baking, digestion, photosynthesis and fireworks, where metal compounds burn with coloured flames.
• Starting a reaction
  • many need activation energy, which is why a match must be struck.
• Catalyst
  • it makes a reaction go faster without being used up.
• Atoms
  • the total number of atoms stays the same because reactions rearrange atoms only.